It has long been a goal of science education in the United States that students leave school with a robust understanding of the nature of science. Decades of research show that this does not happen. Inquiry-based instruction is advocated as a means for developing such understanding, although there is scant direct evidence that it does. There is a gap between what is known about students' inquiry practices and their epistemological beliefs about science. Studies of students' ideas about epistemological aspects of formal science are unlikely to shed any light on how they perceive their own inquiry efforts. Conversely, inquiry-based instruction that does not account for the epistemological beliefs that guide students' inquiry stands very little chance of helping students to understand professional science. This paper reviews largely independent lines of research into students' beliefs about the nature of science and their practices of inquiry to argue that students' inquiry is guided by practical epistemologies that are in need of study. An approach to studying practical epistemologies is proposed that has the potential to produce a better psychological theory of epistemological development, as well as to realize goals of a science education that develops scientifically informed citizens. © 2005 Wiley Periodicals, Inc. Sci Ed, 89:634–656, 2005